Conventional ultrasound imaging systems comprise an array of ultrasonic transducers which are used to transmit an ultrasound beam and then receive the reflected beam from the object being studied. For ultrasound imaging, the array typically has a multiplicity of transducers arranged in a line and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducers can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focussed at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the time delay and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer is employed to receive the reflected sound (receive mode). The voltages produced at the receiving transducers are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focussed reception of the ultrasonic energy is achieved by imparting separate time delay (and/or phase shifts) and gains to the signal from each receiving transducer.
Such scanning comprises a series of measurements in which the steered ultrasonic wave is transmitted, the system switches to receive mode after a short time interval, and the reflected ultrasonic wave is received and stored. Typically, transmission and reception are steered in the same direction during each measurement to acquire data from a series of points along an acoustic beam or scan line. The receiver is dynamically focussed at a succession of ranges along the scan line as the reflected ultrasonic waves are received.
FIG. 1 depicts an ultrasound imaging system consisting of four main subsystems: a beamformer 2, processors 4 (including a separate processor for each different mode), a scan converter/display controller 6 and a kernel 8. System control is centered in the kernel, which accepts operator inputs through an operator interface 10 and in turn controls the various subsystems. The master controller 12 performs system level control functions. It accepts inputs from the operator via the operator interface 10 as well as system status changes (e.g., mode changes) and makes appropriate system changes either directly or via the scan controller. The system control bus 14 provides the interface from the master controller to the subsystems. The scan control sequencer 16 provides real-time (acoustic vector rate) control inputs to the beamformer 2, system timing generator 24, processors 4 and scan converter 6. The scan control sequencer 16 is programmed by the host with the vector sequences and synchronization options for acoustic frame acquisitions. The scan converter broadcasts the vector parameters defined by the host to the subsystems via scan control bus 18.
The main data path begins with the analog RF inputs to the beamformer 2 from the transducer 20. The beamformer 2 outputs two summed digital baseband I,Q receive beams. The I,Q data is input to a processor 4, where it is processed according to the acquisition mode and output as processed vector (beam) data to the scan converter/display processor 6. The scan converter accepts the processed vector data and outputs the video display signals for the image to a color monitor 22.
Referring to FIG. 2, the beamformer is responsible for the transmit and receive beamforming. It includes a probe select switch 26 for activating a selected one of a plurality of transducers. Preferably, each transducer probe assembly incorporates a multiplexer 28 for multiplexing 128 beamformer channels to up to 256 transducer elements. Each transducer includes an array of separately driven transducer elements, each of which produces a burst of ultrasonic energy when energized by a pulsed waveform produced by a transmitter 30. The ultrasonic energy reflected back to the transducer array from the object under study is converted to an electrical signal by each receiving transducer element and applied separately to an analog receiver channel 34 through a set of transmit/receive (T/R) switches 32. Transmitter 30, receiver channels 34 and switches 32 are operated under control of a front end controller (not shown) in the beamformer. A complete scan is performed by acquiring a series of echoes in which switches 32 are set to their transmit position, transmitter 30 is gated ON momentarily to energize each transducer element, switches 32 are set to their receive position, and the subsequent echo signals produced by each transducer element are applied to the respective receiver channels 34.
The receive waveform for each channel is amplified and digitized. The digitized channel signal is then demodulated and filtered by the digital channel and beamforming circuitry 36 to form I and Q baseband signals. These baseband signals are appropriately delayed and pipeline summed to accomplish the steering and focusing of the receive beam. The summed I and Q signals are conditioned by the equalization board 38 to provide an optimal beamformed signal which is output to processors 4, which can perform a variety of calculations on these beam samples, depending on the type of image to be reconstructed.
It is known in diagnostic ultrasound imaging to provide a probe having a number of transducer elements greater than the number of available system receive channels and then time multiplexing the transducer elements outputs into respective receive channels. However, due to the hardware implementation of the beamformer in these systems, complex probe multiplexer switching schemes have not been required. In previous implementations, it was possible to multiplex transducer elements on a channel-by-channel basis, i.e., there was no requirement to multiplex the channels in groups.
One ultrasonic probe which can be used with the system depicted in FIGS. 2 has 192 transducer elements. In contrast, the beamformer has only 128 receive channels. However, a hardware implementation of that beamformer wherein a plurality of 8-channel time delay boards are pipelined does not allow channel-by-channel multiplexing. Instead channels must be multiplexed in minimum group sizes equal to the number of discrete receive channels on the time delay board to ensure that any discontinuous time delays due to non-adjacent transducer elements occur between time delay boards. This implies that only nine unique, 128-channel, multiplexer states are available. This hardware implementation demands additional creativity to control the probe multiplexer in a manner that provides adequate system performance, in terms of vector transition times (i.e., minimum pulse repetition intervals using multiplexed probes), minimal image artifacts (i.e., spectral or structured image artifact), and superior image quality.
Thus, there is a need for a transducer multiplexer control scheme which is compatible with beamforming channels that must be multiplexed in minimum group sizes.